In the past, generally a wrist assembly of an industrial robot has been provided with a first wrist portion secured to the free end of the robot arm so as to be rotatable about the longitudinal axis of the robot arm, or immovably fixed thereto. A second wrist portion is provided between a pair of projections formed at the fore-end of the first wrist portion, and supported on both of the projections of the first wrist portion so as to be rotatable about a first axis perpendicular to the longitudinal axis of the robot arm. A third wrist portion, constructed to allow work attachments such as robot hands and so forth to be secured thereto, is supported on the second wrist portion so as to be rotatable about a second axis perpendicular to the first axis. The second wrist portion and the third wrist portion are driven by electric motors connected thereto through power transmission mechanisms. Generally, it is necessary to carry out the reduction needed through power transmission mechanisms in such a manner that the ratio of the number of rotation of each wrist portion to the number of output shafts of motors may be the predetermined reduction gear ratio.
In the conventional aforementioned wrist assembly, the power transmission mechanism for the second wrist portion is provided with a pair of bevel gears disposed in one projection of the first wrist portion, and the power transmission mechanism for the third wrist portion is provided with a pair of bevel gears disposed in the other projection of the first wrist portion and a pair of bevel gears disposed in the second wrist portion.
However, generally the reduction gear ratio obtained through a bevel gear is limited to about 1/2, and therefore, it is necessary to reduce a rotative power of the motor by a further increase in the number of steps or to incorporate a reduction gear, used exclusively for reduction, with gears incorporated therein, in order to obtain a desired reduction gear ratio by the above-mentioned power transmission mechanism. However, this leads to an increase in the weight of the wrist assembly and an increase in the number of parts.